Continuous combustion engines such as gas turbines are powered by the continuous combustion of fuel/air mixtures in predetermined relative proportions, accompanied by the continuous delivery of the products of the combustion, including highly objectionable nitrogen oxide gases, primarily NO and NO.sub.2, generally referred to collectively as NO.sub.x, The extent of formation of such NO.sub.x species is to a large extent controlled by the chemical kinetic rate of their formation during the combustion process and the time scale of critical steps in the combustion process. The kinetic rate is dependent upon the fuel-to-air ratio, which generally varies in different areas of the combustion chamber, and the temperature in said areas. The combustion process time scales are dependent upon fuel and air input and mixing conditions.
The liquid fuel is introduced through nozzles or fuel injectors in the form of a fine spray. The air is compressed and introduced to the combustion chamber through a multiplicity of discrete jets and cooling passages. A minor amount of the air passes through the fuel nozzle to assist spray formation and distribution. A further amount of the air is introduced to mix with this spray as primary air to form an initial combustible mixture with the fuel. The bulk of the air supply flows into the combustion chamber further downstream, in part as cooling flow for the combustor walls, and in part as directed jets forcing mixing of the flow in the combustor. It completes combustion of the gases from the primary combustion zone, and dilutes and cools them.
In the case of gas turbine engines, the temperature of the combustion air is very high and this favors the rapid formation of NO.sub.x gas species. Such pollutants result in unhealthy air conditions at ground level in the area of airports and contribute to ozone depletion at higher altitudes.
The overall problem is to limit or minimize nitrogen oxide production and attendant emissions from combustors, particularly gas turbines, having combustion systems firing clean liquid fuels (e.g. kerosene, JP-4, Jet-A) with air. In gas turbines, the primary fuels are Jet-A and JP-4, with air typically compressed to a pressure ratio in the range 6 to 50. At sea level, this results in air input to the combustor at 500.degree.-1200.degree. F., and operating pressure in the range 6 to 50 atmospheres. The global combustion process typically includes substantial excess air.
Under these conditions, fixation of air nitrogen, primarily as nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2), is thermodynamically driven. Chemical equilibrium NO.sub.x (NO+NO.sub.2) concentration is dependent on initial air temperature and combustion stoichiometry. The actual quantity of NO.sub.x is typically less than this equilibrium value, controlled by the rate of its formation during the combustion process.
Typically, a gas turbine combustor exit stoichiometry is in the range 0.3 to 0.5 (F/A), with corresponding equilibrium NO.sub.x in the range 40 to 3500 ppm--see FIG. 1 of the accompanying drawings.
This is not acceptable in terms of atmospheric pollution. Stationary power systems and low altitude aircraft applications result in concentrated NO.sub.x emission, typically near urban areas. Current-generation commercial aircraft, cruising in the upper troposphere, add NO.sub.x in the 25,000-40,000 foot altitude range. Higher flight altitude, attractive for supersonic cruise, will result in NO.sub.x dispersal in the stratosphere, resulting directly in destruction of atmospheric ozone (O.sub.3) by selective reaction with NO.sub.x.
NO.sub.x formation is based on complex free radical gas phase chemistry. In the course of combustion of hydrocarbons with air via a spray diffusion flame subject to dilution with excess air, the reacting mixture composition and temperature inevitably transitions through a regime of very high NO.sub.x formation rate. This problem is exacerbated by the high combustion air temperature associated with gas turbine operation.
This rate varies with input air temperature and local combustion gas stoichiometry, and can be expressed in ppm/millisecond--see FIG. 2 of the accompanying drawings. Relative to a total residence time of typically 5 msec, peak formation rates of a few ppm/millisecond are acceptable. However, realistic values of gas turbine combustor input air temperature result in peak formation rates in the range 100 to 1000 ppm/msec. Following the sequence of fuel oxidation from high to low stoichiometry, the absolute equilibrium NOx level is low for stoichiometry greater than about 1.4, even though the formation rate approaching this level can be high. Transitioning to lean conditions, at stoichiometries in the range 0.5 to 0.6 the formation rate becomes low enough that even though the equilibrium level can be high, it is not achieved in the available combustor residence time. Consequently, the critical regime for NO.sub.x formation is in the F/A composition transition from about 1.2 to 0.6.